1. Field of the Invention
The present invention relates to computer aided design (CAD) methodologies used to implement integrated circuits, and more specifically to a method and apparatus to reduce computational complexity in characterization of various parameters representing cross-talk noise of cells in a library used in designing integrated circuits.
2. Related Art
Libraries are often used in designing integrated circuits (ICs). In general, a library contains multiple cells (e.g., logic gates, multiplexers, etc.), and a designer selects and includes a desired cell in a circuit design. Several libraries are often provided, each being designed for a corresponding combination of manufacturing process, temperature, voltage (PTV), etc.
Page of Cells are often characterized prior to being used in the design on ICs. Characterization generally refers to the determination of various electrical parameters of interest for each cell. Such pre-determination enables a system (typically, using which an IC is being designed) to quickly analyze a proposed design for conformance with various criteria (e.g., design rules, timing constraints) as a designer designs a desired circuit.
Cross-talk noise among nets connecting cells is often of concern in the design of ICs, particularly in deep submicron technologies. Cross-talk noise generally refers to interference caused by some nets (aggressor) on other nets (victims). Of particular concern in IC designs is crosstalk noise induced due to parasitic coupling. Such induced noise or glitches could cause functional failure by switching logic_states. In the case of designs employing dynamic logic design styles, this problem is more severe due to increased noise sensitivities of pre_charged nodes.
By analyzing a (proposed) circuit for the effects of such cross-talk noise, one may ensure that a later fabricated circuit would operate in a desired manner even in the presence of cross-talk noise. At least for such a reason, it is generally desirable that cells in a library be characterized to determine various related parameters (“noise parameters”).
One noise parameter of interest is ‘noise immunity’. Noise immunity parameters generally indicate whether a glitch of a given width and/or height on an input pin of a cell would cause the signal at an output pin (and thus the connected nets) to be altered by more than a pre-specified threshold (and/or a change of state in the case of sequential elements). Often a noise immunity curve (NIC) is generated which indicates the minimum height of an input glitch that would cause an output glitch exceeding a pre-specified threshold level/height for a given width of an input glitch.
Another noise parameter of interest is ‘noise propagation’ (NP), which specifies the characteristics of an output glitch in response to a input glitch of a given height and width. NP curves (commonly referred to as NP characteristics) are often drawn mapping the height (peak) of the input glitch to the height of the output glitch. The NP curves would indicate the effect of an input glitch (on one cell) on the subsequent cells connected to the output net of the cell.
The characterized NIC and NP curves are used to quickly determine failures in response to input glitches in integrated circuits. For example, in static noise analysis (SNA), the NP characteristics/curves and NICs are used to determine the effect of aggressor nets on victim nets. Often the analysis is performed without regard to the states of the input pins generally because the probability that the states of all pins will be known is low. The results of the analysis are used as a basis to determine which nets need to be redesigned. By pre-characterizing the curves, such analysis may be performed quickly.
The need for such characterization is increasing with improvements (e.g., from 130 nanometer process technology to 90 nanometer process technology) in fabrication technologies, which are enabling packing of more transistors into the same area. It has been observed that the coupling capacitance contributes to a greater degree (as a fraction of the total capacitance on a net) with such improvements. Accordingly, an increasing need is believed to be presented to characterize the related noise parameters.
One general requirement in characterization of cells for the noise parameters often is to reduce the computational requirement. Often due to a large number of cells in each library and/or a large number of libraries (e.g., to correspond to different PVT combinations), it may take several days or weeks to characterize the noise parameters of the cells in cell libraries. By decreasing such total characterization time, the total time to design an IC as well as cost for characterization, can be reduced.
What is therefore needed is a method and apparatus to reduce computational complexity in characterization of various parameters representing the effect of cross-talk noise on cells in a library used in designing integrated circuits.